We propose to develop electrophysiologic measures to discriminate depressed older patients with treatment-responsive cognitive deficits from depressed older patients with progressive, neuropathologically-based dementia. Although depression is common among the elderly, we are challenged to reliably distinguish late-life depression with associated reversible cognitive deficits from primary degenerative dementia at the time of initial evaluation. Furthermore, we do not yet know conclusively if clinical course and treatment response are associated with age of onset of the depressive illness, although it appears likely that a more severe, irreversible type of depression can have its onset later in life. In order to better describe late-life depression and to test for differences associated with age of onset, we will obtain neuropsychological and brain electrophysiologic measures of cognitive processing in 15 early-onset (before age 50) and 15 late-onset DSM-III diagnosed depressives who are at least 50 years old. As control subjects, we will study 15 patients with primary degenerative dementia and 15 normal controls who are matched to the depressives in terms of age, sex, and education. Cognitive functions will be assessed at repeated intervals (pre- and post-treatment for depressives) using standard neuropsychological and event-related potential (ERP) measures. All depressed patients will receive the same standard treatment as part of a natural history study currently underway in the Department of Psychiatry. Study of the latency of ERP components will supplement knowledge derived from behavioral measures by adding information regarding the timing of evaluation and decision processes. We will test the prediction that late-onset depressives are more likely to suffer from severe cognitive deficits of the type known to exist in primary degenerative dementia than are early-onset depressives. We will also test the prediction that later-onset is associated with poor treatment response. This initial study represents the first year of a 3-5 year longitudinal project, at the end of which we aim to identify predictors of clinical course and treatment response in geriatric depression.